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Climate Change and Ocean Systems: Introduction and Background

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The ocean moderates climate change at the cost of profound alterations of its physics, chemistry, ecology, and services. However, despite the ocean’s critical role in global ecosystem processes and services, international climate negotiations have only minimally touched on ocean impacts. Any new climate regime that fails to minimize ocean impacts will be incomplete and inadequate. This session, as well as session "2207: Ocean Change: Understanding and projecting the impacts of warming and acidification on natural and human systems" on Wednesday afternoon will provide an integrated and updated perspective on the changes, risks and projections for both natural and human systems. This will facilitate the construction of key messages for the COP21 negotiation process on the Ocean and associated issues.

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Challenges to ocean life and associated human interests: IPCC assessments and beyond

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Oceans cover more than 70% of the planet and their biota create half the oxygen (O2) produced on the planet. Oceans cover about 11 % of the global population’s demand for animal protein. Climate change causes oceans to warm and stratify, and sea level to rise, as well as Arctic summer sea ice to shrink. Ocean warming accounts for more than 90% of the energy accumulated in the climate system. Warming causes oceans to lose oxygen overall and during an expansion of hypoxic water layers. Concomitantly, the accumulation of anthropogenic CO2 in ocean surface waters disturbs water chemistry and causes acidification. These climate drivers alter ocean ecosystems and the services they provide. They frequently relocate and reduce marine biological resources on which human societies depend, affecting economic benefits, livelihoods, food availability and public health particularly for coastal communities.

The recent IPCC assessment report (AR5) as well as the Structured Expert Dialogue have comprehensively considered impacts, vulnerability, adaptation options and projected climate risks for the oceans and their services to humankind (www.ipcc.ch). Ocean warming has caused geographical shifts in the distribution of marine species, associated with changes in the species composition and function of ecosystems. Recent meta-analyses indicate that ambient temperature and hypoxia extremes in some regions are already close to permanent tolerance limits of marine animals and plants indicating a risk of expanding water bodies void of higher marine life. Empirical observations together with mechanism-based knowledge of organism and ecosystem vulnerabilities support the detection of climate impacts in the field and their attribution to climate change. They also support more accurate, higher confidence projections of climate change impacts in the oceans, as well as of associated risk such as of ocean acidification effects on key animal phyla and their economic value. Such assessments of risks were recently developed further by the Oceans 2015 initiative.

Warming-induced shifts in ocean productivity and species distribution, including of exploited fish and invertebrates, and declines in their body size are projected to result in reductions of fisheries productivity, especially at lower latitudes. In contrast, fisheries at high latitudes may benefit from increased abundance and diversity of commercially valuable species. Evidence is increasing that anthropogenic ocean acidification is affecting organisms, ecosystems and associated human interests (particularly bivalve fisheries and aquaculture) in areas with and without a natural background of elevated CO2 concentrations. Hypoxic areas that exclude active pelagic fishes such as tuna and their fisheries are expanding. Impacts thus go beyond those of simply warming and include effects of acidification and deoxygenation. These combined effects of the three climate drivers will lead organisms to reach long-term tolerance limits even earlier than with temperature changing alone, enhancing sensitivity through dynamic shifts of thermal limits. For example, recent modeling emphasizes that combined warming and oxygen loss constrain metabolic scope of key species and thereby habitat and biogeographical distribution across wider ocean areas than previously thought.

Through effects on performance at the levels of reproduction, behaviour and growth, marine life forms including those that are economically relevant, are thus threatened by climate drivers changing individually and even more so by their additive or synergistic effects. Together with shifts in ocean circulation and productivity the resulting dynamic changes in thermal bioenvelopes have major implications for the ranges of geographical distribution of marine species, their competitive and trophic interactions, population dynamics and community compositions. Such integrative view is also being developed for paleo-observations of climate change effects. It should also be included in modeled projections of ecosystem change, which will inform social-ecological models projecting effects on fisheries and aquaculture.

Assessments of risks of ocean warming, deoxygenation, and acidification to ecosystem services have generally concluded that human communities and artisanal fishermen at low latitudes often have the lowest capacity to adapt to losses in natural resources, for example by replacement with alternative foods. Substantial challenges remain to anticipate the ecosystem-wide impacts of the combined drivers of warming, deoxygenation, and acidification, and the ensuing alterations of ecosystem services for human communities.

Finally, the projections of climate change impacts in the oceans and along coasts, including sea level rise, and the associated risks and scopes for adaptation, need to be considered when setting the long-term global goals (LTGG) of climate change mitigation. As an example, the long-term risks of sea-level rise, Arctic sea ice loss and combined impacts of ocean warming and acidification on key groups of corals, bivalves and other calcifiers, that have high economic value in tourism, coastal protection and fisheries, strongly support setting the LTGG to but not above 1.5°C global warming above preindustrial values.

Oceans of Concern

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Oceans of Concern

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Climate change and ocean acidification will reorganize food webs and alter ecosystem function, with attendant impacts on human communities and activities. The impacts of climate change have been detected across all oceans; changes to ocean have altered the timing of plankton blooms and migratory patterns and spawning in fish and invertebrates, over recent decades. Shifts in species distributions to higher latitudes are a commonly reported response of marine fishes and invertebrates to warming oceans. Differences in rates of change with climate change amongst species and populations imply that marine ecosystems may be substantially reorganized at regional scales. Indeed, at a global scale a strong warming signal is already evident in fish catches, with an increasing dominance of warm-water species found in catches coastal and shelf areas. Modelling approaches project a global redistribution of marine biodiversity this century with regional differences driven by species invasions and local extinctions. The emergence of novel assemblages of species with no past or contemporary analogues will consequently require new strategies for managing coastal areas and fisheries. Large changes in species composition are projected in both polar and tropical oceans with increases in species richness at high latitudes. High local extinctions are projected in equatorial oceans, particularly in the central Indo-Pacific, as species are eliminated with warming. The different futures emerge for tropical biodiversity by 2050s under high and low emission scenarios with widespread species losses and declines in species richness around the equator, particularly in the central Indo-Pacific, if global temperatures exceed the 2°C limit. Further, under the medium to high emission scenarios, ocean acidification poses substantial risks to marine ecosystems, particularly coral reefs and polar ecosystems. Climate change is a risk food resources, coastal livelihoods, and industries dependent on the Ocean, adding to the threats of over-fishing and other non-climate stressors, and the likelihood of exceeding adaptation limits increases with greater rates and magnitude of climate change.